133

Diversity of Hydrothermal Systems on Slow Spreading Ocean RidgesGeophysical Monograph Series 188Copyright 2010 by the American Geophysical Union.10.1029/2008GM000756

The Relationships Between Volcanism, Tectonism, and Hydrothermal Activity on the Southern Equatorial Mid-Atlantic Ridge

C. W. Devey,1 C. R. German,2 K. M. Haase,3 K. S. Lackschewitz,1 B. Melchert,1 and D. P. Connelly4

The Mid-Atlantic Ridge south of the equator is a key region for many aspects of spreading axis studies, from biogeography to ridge-hotspot interaction. Despite this, the ridge axis had, until 2004, seen little systematic study. Repeated trips to the area since then have mapped and explored some 900 km of ridge length, from 2° to 14°S. The result is complete bathymetric and side-scan coverage of the axial region and the discovery and characterization of the first hydrothermal vents south of the equator. Such multisegment detailed and interdisciplinary coverage allows us to formulate a general model for the interplay between volcanism, tectonics, and hydrothermalism on a slow spreading ridge. The model defines three basic types of ridge morphology with specific hydrothermal characteristics: (a) a deep, tectonically dominated rift valley where hydrothermalism is seldom associated with volcanism and much more likely confined to long-lived bounding faults; (b) a shallower, segment-center bulge where a combination of repeated magmatic activity and tectonism results in repeated, possibly temporally overlapping periods of hydrothermal activity on the ridge axis; and (c) a very shallow axis beneath which temperatures in all but the uppermost crust are so high that deformation is ductile, inhibiting the formation of high-porosity deep fractures and severely depressing hydrothermal circulation. This model is used together with satellite-derived predicted bathymetry to provide forecasts of the best places to look for hydrothermal sites in the remaining unexplored regions of the South Atlantic.

1Leibniz Institute of Marine Sciences at the University of Kiel (IFM-GEOMAR), Kiel, Germany.

2Woods Hole Oceanographic Institution, Woods Hole, Massa-chusetts, USA.

3GeoZentrum Nordbayern, Erlangen, Germany.4National Oceanography Centre, Southampton, UK.

1. INTRODUCTION

For studies of spreading axes, in general, and their hydro-thermal systems in particular, the Atlantic Ocean occupies a

key position. It exhibits ultraslow (in the obliquely spread-ing Norwegian-Greenland Sea) to slow spreading, with the spreading rate increasing monotonically southward. The ridge axis shows a large variety of depths and across-axis morphologies, from axial volcanic ridges with no or only poorly developed axial graben [Keeton et al., 1997; Smith et al., 1995] to a deep axial valley associated, at ridge-transform intersections and elsewhere, with striated “megamullion” features [e.g., Cann et al., 1997; Smith et al., 2008]. Sev-eral hydrothermal fields are known from the northern Mid-Atlantic Ridge (MAR) extending from the Soria Moria field (R. Pedersen, personal communication, 2008) at around 71°N, north of Jan Mayen (although indications for hydrothermal plumes in the water column have been found much further north [e.g., Connelly et al., 2007; Edmonds et al., 2003])

134 SOUTH ATLANTIC HYDROTHERMALISM

to the Logatchev field at 15°N [Lalou et al., 1996]. South of the equator, though, little was known of hydrothermal activity until both British and German programs began to focus on the area starting in 2002. Here we present a syn-opsis of the results from these programs, detailing what is known about hydrothermal activity south of the equator in the Atlantic and its relationship to volcanology and tecton-ics of the axis. We show various systematic relationships between these features and make some predictions for the location of hydrothermal systems south of the area presently studied.

2. TEcTONIcS ANd VOLcANOLOGy OF ThE SOUTHERN EqUATORIAL MID-ATLANTIC RIDGE

The morphology of the ridge axis from 2° to 14°S is shown in Plate 1. In common with other areas of the MAR, the axis shows highly variable axial morphology. From 2° to 4.5°S, three segments with well-developed axial valleys, separated from one another by short transform faults, are distinguish-able. South of this, around the newly discovered Turtle Pits hydrothermal field, the ridge axis is characterized by a with-in-axis high (see Plate 2) producing an hourglass-shaped segment (water depth at segment ends ~4500 m, at segment center ~3000 m [Haase et al., 2007]). Side-scan sonar im-aging and seafloor ground-truthing observations reveal ex-tensive, virtually unsedimented lava flows, consistent with geophysical evidence of recent seismic activity in the area, all suggesting that this segment is magmatically very active. Active seismic studies on the axis around Turtle Pits [Plan-ert et al., 2009] have shown evidence both for a low-velocity zone in the crust below the median valley and for crust more than 8 km thick on the in-axis high, features which further support the suggestion that this segment is presently mag-matically robust.

To the south of the 5°S Fracture Zone, two second-order segments are probably distinguishable (see Plate 1 and Ger-man et al. [2008a]) before the large Ascension Fracture Zone is reached. The northernmost of these segments has recently split an uplifted massif originally formed as an inside corner high adjacent to the 5°S Fracture Zone (see Plate 2 and Reston et al. [2002]). hydrothermal prospecting with conductivity- temperature-depth (cTd) casts on these segments at 5°30.6′S/11°31.5′W and 6°06.6′S/11°26.81′W, and con-tinuous towed ocean bottom instrument (TOBI)/miniature autonomous plume recorders (MAPR) surveys along the length of this section have revealed no evidence of any hy-drothermal signals [German et al., 2008a].

The Ascension Fracture Zone is a double fracture zone with a short spreading segment between the fractures (Fig-ure 1). Large areas of the seafloor between the fracture zones

are characterized by relatively smooth topography and ridge-perpendicular striations on the seafloor (shown by field on Figure 1). These features are a typical bathymetric expression of so-called “megamullions” [Cann et al., 1997], thought to be the product of largely amagmatic extension along detach-ment faults (a feature of slow spreading axes, in general, and this part of the spreading system, in particular [see Reston et al., 2002]). Sampling of this striated surface (for sampling localities, see Figure 1) yielded a cataclastite consisting of serpentinite and altered basalt at station 123KDS and small amounts of gabbro showing signs of brittle deformation at station 124KDS [Schulz et al., 1999], confirming that rock types more typical of deeper crustal regions are exposed on the seafloor on this striated surface.

The segments south of the Ascension Fracture Zone, which were numbered A1 to A4 by Bruguier et al. [2003], vary greatly in their tectonics and volcanology. Segment A1 is characterized by a deep axial valley (mean depth ~4000 m with flanks at ~2000 m) in which numerous isolated volca-nic edifices have been identified on TOBI side-scan images (see Plate 3 for an interpretation of the side-scan information from one section (location marked on Plate 1) of this seg-ment). The axial valley itself is floored mainly by hummocky lava terrain (although some volcanic cones with a central caldera are also seen, see Plate 3), with some lava flows cut by fault scarps. Fault scarps are prominent throughout the entire length of the A1 axial valley. Segment A2 (Plate 4) has a somewhat narrower axial region floored by unsedi-mented hummocky terrain; this area shows, however, far fewer faults than the hummocky terrain on segment A1. The A2 axis is, in contrast to the A1 axis, located on a distinct ba-thymetric high. Near the center of A2, a prominent lava flow surrounds numerous volcanic cones and volcanic ridges; this flow is cut by only a few fault scarps and will be the subject of further discussion later. Generally, the level of side-scan reflectivity on segment A2 is higher than on A1, suggesting less sediment cover and fresher lavas on the seafloor on A2.

Segment A3 is both the shallowest and also apparently the most volcanically active of the segments between the As-cension and Bode Verde Fracture Zone. The segment has no axial valley, but instead, the neovolcanic zone sits on the crest of a broad axial high. Side-scan studies show extensive fresh lava flows within and around the axial region and far fewer areas of hummocky lava texture or tectonic structures than on segments A2 or A1. Several isolated volcanic cones are also visible (Plate 5).

There is a strong contrast both volcanically and tectoni-cally between the shallow, volcanically dominated segment A3 and segment A4 to the south. Segment A4 has a deep axial valley similar to A1 (compare Plates 3 and 6), although the valley floor at A4 is generally not as wide as A1. Several

Plate 1. Bathymetry and location of spreading axis (green line) in the region 2°–14°S in the Atlantic. Also shown are the segment names given by Bruguier et al. [2003]. conductivity-temperature-depth (cTd) stations that have been occupied on the axis are shown as gray circles (note: the few stations falling off the axis were for background and calibration pur-poses, shown here for completeness). Red labeled stars are the hydrothermal sites so far discovered. Boxes show areas covered by subsequent plates and figures. Map created using GeoMapApp: most bathymetric information is from satellite altimetry data [Smith and Sandwell, 1997] with higher-resolution information from the LDEO marine geophysics data bank cruises RC2515, 2601, and 2602 carried out in the 1980s.

136 SOUTH ATLANTIC HYDROTHERMALISM

linear volcanic ridges (e.g., Plate 6) marked by hummocky terrain characterize the valley floor; these ridges are sepa-rated and flanked by heavily tectonized and/or sedimented areas. The volcanic ridges are offset from one another, lead-ing to at least three third-order subdivisions [MacDonald et al., 1988] of the A4 segment (at 10°42′S, 10°50′S, and 11°10′S). The southernmost of the third-order segments has a well-characterized hydrothermal plume signature in total dissolved Fe and Mn (TdFe and TdMn) concentrations [German et al., 2002].

3. RECENTLY DISCOVERED HYDROTHERMAL SITES

As seen already on Plate 1, during the last 4 years, the first three hydrothermal sites south of the equator in the Atlantic have been found and studied. Evidence from water-column

work for the presence of two additional sites, at ~4°S (from TOBI/MAPR data [German et al., 2008a]) and 8°17′S (from TDMn in a water sample [German et al., 2002]), has also been gathered but as the tectonic settings of these systems have not been unequivocally determined by direct mapping and obser-vation of the fields, they will not be discussed further here.

The discovery of all hydrothermal sites was the result, pri-marily, of systematic hydrothermal prospection in the water column along the ridge axis, investigating both chemical and physical indications of the presence of hydrothermal plumes. In many cases, this prospection was performed on a regional scale with the deep-towed TOBI side-scan vehicle fitted with MAPR backscatter sensors suspended both above and below the deep-towed vehicle (for a description of methods, see German et al. [2008b]), augmented by local deployments of a CTD rosette sampler. While the information from the CTD device itself can provide indications of hydrothermal plumes

Plate 2. Bathymetry of the area around 5°S from Planert et al. [2009]. Also shown are the positions of seismic lines shot in 2002, interpreted by Planert et al. The Turtle Pits site was discovered at the position where Profile 11 crosses the median valley and is marked. Also marked is the location of the split inside-corner high studied by Reston et al. [2002]. For regional setting of Plate 2, see Plate 1.

DEVEY ET AL. 137

Figu

re 1

. Bat

hym

etry

, sam

ple

loca

tions

, and

geo

logi

cal i

nter

pret

atio

n of

the

Asc

ensi

on F

ract

ure

Zone

are

a fr

om M

ölle

r [2

002]

. Bla

ck li

nes m

ark

spre

adin

g ax

es, s

olid

whi

te li

nes a

re a

ctiv

e tra

nsfo

rm fa

ults

, and

whi

te d

ashe

d lin

es th

e fr

actu

re

zone

trac

e. T

he w

hite

dot

ted

line

encl

oses

the

appr

oxim

ate

area

cov

ered

by

stria

ted

seafl

oor.

Labe

led

cros

ses

are

site

s dr

edge

d du

ring

crui

se M

41/2

. Bat

hym

etry

pro

cess

ing

by N

eil M

itche

ll. T

he re

gion

al lo

catio

n of

Fig

ure

1 is

sho

wn

in

Plat

e 1.

138 SOUTH ATLANTIC HYDROTHERMALISM

through the disturbance they cause in the regional layering of water masses (visible on a T-S diagram, for example), it is the analyses of water samples collected with the Niskin bot-tles on the rosette or additional sensors attached to the rosette, which yield the most information. A brief description of the sites known to present is given, from north to south, below.

3.1. Site 4°48′S: Turtle Pits, Red Lion, Comfortless Cove, and Wideawake

The first indication for hydrothermal activity at the 4°48′S site came from deep-towed turbidity profiles (as part of a 2005 TOBI/MAPR side-scan sonar and turbidity survey). As a result of this work, it was possible to plan autonomous un-derwater vehicle (AUV) deployments (using the ABE vehicle from Woods hole Oceanographic Institution), which lead to the localization and visual observation of the Turtle Pits site (using a method described by German et al. [2008b]). Fol-lowing that initial localization, the Turtle Pits area was the subject of extensive studies [Haase et al., 2007] and has been found to produce a strong turbidity signal in the water column, which reaches up to 300 m above and up to 4 km away from the vent site itself [Bennett et al., 2008; Devey et al., 2008; Koschinsky et al., 2006]. Oceanographic experi-ments are underway (M. Walter, personal communication, 2008) to attempt to quantify the vent output at the Turtle Pits site. Of the known vents, Turtle Pits and Comfortless Cove discharge very similar volatile-rich, low-chlorinity fluids at temperatures up to 407°C, which are thought to be the vapor-rich part of a phase-separated system [Haase et al., 2007; Koschinsky et al., 2008]. Red Lion, which is also a black smoker system, discharges, in contrast, fluids with seawater- like chlorinities at temperatures up to 348°C [Haase et al., 2007; Koschinsky et al., 2008] and so apparently has a sepa-rate fluid supply to that which is feeding Turtle Pits and com-fortless cove. Wideawake, the southernmost field, shows only diffuse venting; calculations of the end-member fluid compositions show that it is also most likely fed by Turtle Pits-type fluids. close to and indeed covering part of the Wideawake Field is a fresh lava flow. This flow has been mapped with AUV and remotely operated vehicle (ROV) dives; initial information on the AUV mapping was presented by German et al. [2008a], the final map including all pres-ently available information is shown in Plate 7. It has been suggested [German et al., 2008a] that this flow may have been erupted during a seismic crisis in the area in June 2002.

3.2. Nibelungen Field

The Nibelungen Field is located at the foot of an east- facing fault scarp, some 9 km to the east of segment A2 and

just to the south of the nontransform offset separating seg-ments A1 and A2 [Melchert et al., 2008]. The field itself is characterized by one solitary black smoker vent: no evi-dence for any other present-day hydrothermal activity has been seen. The Nibelungen fluids are characterized by high CH4/3He ratios [Keir et al., 2008], implying a heat source not related to active magmatism [Melchert et al., 2008]. These authors have suggested that this field is the first to have been found that is “mining” heat from hot deep-crustal or mantle rocks.

3.3. Lilliput

The low-temperature Lilliput field lies on the axis of the inflated, shallow segment A3 at ~9°30′S [Haase et al., 2009]. In total, four diffuse vents between 9°32.5′ and 9°33.2′S have been found, apparently associated with the location of a recent eruption and diking event. The area is also charac-terized by evidence for lava ponding similar to that seen on fast spreading or magmatically active slow spreading ridges [e.g., Ondréas et al., 2009; Soule et al., 2005].

4. REGIONAL HYDROTHERMAL PROSPECTION

Hydrothermal prospection, which led to the discovery of the sites outlined above, has been carried out along much of the ridge axis between 4° and 11°S. The prospection was designed to detect any neutrally buoyant plumes associated with black smoker activity in the ridge valley [Devey et al., 2005; German et al., 2008a] by using the deep-towed TOBI vehicle in combination with MAPR attached to cables both above and below TOBI. Previous work on plume dispersal around Rainbow [German et al., 1998], where the topo-graphic setting is similar to most of the axial valley studied here, shows that its neutrally bouyant plume rises some 300 m above the seafloor and has a minimum width (perpendicular to the current-induced dispersal direction) near its source sim-ilar to the Rossby radius, i.e., 3 km. Further from the source, the plume widens appreciably, reaching 10-km width at 20 km from the vent as detected by backscatter sensors similar to those used during our survey. German et al. [1998] and other workers have shown that the plume itself is always ≥100 m thick and rises 200–300 m above its source on the seabed. Thus, our survey strategy using one or two TOBI tow-lines within the 10-km-wide axial valley, towing at ~300 m above the seafloor and with MAPR mounted at least 100 m below and 100 m above the vehicle should intersect all plumes present in the valley. The results of such a MAPR survey are shown in the Figure 3 of German et al. [2008a]. Between 4°S and the Ascension Fracture Zone, this prospection found the systems detailed in German et al. [2008a]. Between the As-

DEVEY ET AL. 139

Plat

e 3.

(rig

ht) B

athy

met

ry, (

mid

dle)

TO

BI s

ide-

scan

dat

a, a

nd (l

eft)

thei

r int

erpr

etat

ion

for a

por

tion

of th

e ax

ial v

alle

y of

seg

men

t A1.

The

loca

tion

of P

late

3 is

sho

wn

in P

late

1. T

he in

terp

reta

tion

of th

is a

nd o

ther

TO

BI s

ide-

scan

imag

es

pres

ente

d he

re is

bas

ed o

n an

alys

is o

f the

aco

ustic

faci

es in

a m

anne

r sim

ilar t

o th

at u

sed

by S

aute

r et a

l. [2

002]

.

140 SOUTH ATLANTIC HYDROTHERMALISM

Plat

e 4.

(rig

ht) B

athy

met

ry, (

mid

dle)

TO

BI s

ide-

scan

dat

a, a

nd (l

eft)

thei

r int

erpr

etat

ion

for a

por

tion

of th

e ax

ial v

alle

y of

segm

ent A

2.

DEVEY ET AL. 141

cension and Bode Verde Fracture Zones, no neutrally bouyant plumes were found in the axis with the TOBI-MAPR com-bination [Devey et al., 2005]. Subsequently, intensive CTD and AUV work lead to the discovery of two systems, which the TOBI + MAPR combination in the axis cannot detect, the Nibelungen system in an off-axis setting and the low-temper-ature diffuse Lilliput site, which produces no bouyant plume.

Plate 8 shows a map summarizing the hydrothermal pros-pecting carried out between 7° and 11°S to date, with insets showing details of positive signals around the heavily stud-

ied vent areas. As has already been reported [Devey et al., 2005], the Niblungen site shows a clear turbidity anomaly in the water column, some 100–300 m above the seafloor, a typical situation for a black smoker vent. Lilliput, on the other hand, shows only the smallest of turbidity signals (see inset in Plate 8, the one profile showing a systematic signal is only affected in the lowermost 5 m), which is in agreement with a low-temperature venting site (no turbulent plume) situated in relatively shallow water (low-temperature fluids containing little suspended material). Early work on

Plate 5. (right) Bathymetry, (bottom) TOBI side-scan data, and (top) their interpretation for a portion of the axial valley of segment A3. The star shows the location of the Lilliput hydrothermal field. The regional setting is shown in Plate 1.

142 SOUTH ATLANTIC HYDROTHERMALISM

Plate 6. (bottom) Bathymetry, (left) TOBI side-scan data, and (right) their interpretation for a portion of the axial valley of segment A4. The location of the area is shown on Plate 1.

DEVEY ET AL. 143

plume detection [Baker et al., 1985] on the Juan de Fuca axis (which has a similar axial morphology to segment A2) showed that the particle plume can be detected over 100 km from the axis. The intensity of water column work carried out over segment A2 (see Plate 8) makes it, therefore, highly unlikely that undetected high-temperature hydrothermal sys-tems still remains to be found on this segment. The same is probably true for segment A3 at least as far south as 9°30′S, as, even though high-temperature venting on shallow ridges may inherently occur at lower temperatures and hence pro-duce less particle-rich plumes, the dissolved gas signature of this venting should have been detected. The southern end of segment A3 and almost the whole length of segment A4 remain virtually unexplored.

5. WHY ARE HYDROTHERMAL SYSTEMS WHERE THEY ARE?

The study region encompasses three first-order ridge discontinuities (5°S, Ascension and Bode Verde Fracture Zones) and numerous second-order offsets. The relatively

intensive search for hydrothermal systems and the detailed study of their volcanotectonic setting that have been carried out in this area allow us to investigate the relationships be-tween volcanism, tectonics, and hydrothermalism on a mul-tisegment scale.

An initial observation is that all three hydrothermal sys-tems are, as are hydrothermal systems almost everywhere [Curewitz and Karson, 1997], associated with fractures vis-ible at the surface. At Turtle Pits and Lilliput, these fractures are close to and similarly orientated to the pathways followed by the recent volcanic activity. It is this recent magmatic ac-tivity that presumably provides the relatively shallow heat source for the hydrothermal activity. At Nibelungen, no evi-dence for recent magmatic activity has been found either on the seafloor, which is heavily sedimented, or in the fluids, which are 3He-poor, and so no direct relationship between active magmatism and hydrothermal activity, which is uni-versally applicable, can be proposed for the region.

Fractures are also the major source of permeability in the deeper crust [Carbotte and Scheirer, 2004] and permeabil-ity is the key factor governing the location, intensity, and

Plate 7. The results of visual mapping of the seafloor around the Wideawake lava flow superimposed on detailed ba-thymetry (contour interval 2 m) collected by “ABE.” Based on data from both “ABE” automated photo-survey (dive 174) and from remotely operated vehicle Kiel6000 dive tracks. The flow shown in red is the fresh lava, which has been pro-posed [German et al., 2008a; Haase et al., 2007] to be related to the 2002 seismic swarm in the area. M marks the position of mussel beds which appear to occur only on the unsedimented, glass-free lava.

144 SOUTH ATLANTIC HYDROTHERMALISM

temperature of hydrothermal circulation [Wilcock, 1998]. Surface fractures are by no means a clear indicator for hy-drothermal activity; however (as has also been shown to be the case on the superfast East Pacific Rise [Hey, 2004]), the side-scan studies of the ridge crest and ridge offsets over the entire area show that, with the possible exception of the summit region of segment A3, these are all heavily fractured (see, e.g., Plates 3–6) even where no hydrothermal signal has been found. So why are some fractures the location of

hydrothermal effulent and others not? In order to answer this question, information about where hydrothermal activ-ity is not present is as important as information on where it is presently occurring. As we have seen previously, de-spite extensive searching, we have found no evidence for high-temperature hydrothermal activity along the rift-crest of either segment A1 or A2, even though some indications had been deduced from earlier CTD work [German et al., 2002]. This contrasts strongly with the geological informa-

Plate 8. Details of hydrothermal prospection work carried out on segments A2 and A3. Dots show location of water column profiles (either MAPR or cTd), red dots mark stations where clear evidence for hydrothermal activity (optical backscatter or CH4 anomalies in the water column) were found. Inset boxes show details of ch4 concentration results near (top) Nibelungen and the reduction of light backscatter near the seafloor in the one station which showed this signal close to Lilliput (bottom, anomaly circled in red). The bathymetry data for the main map comes from the GeoMapApp applet and has the same depth scale as Plate 1. The bathymetry data for the upper inset was collected during cruise Meteor 62/5; the bathymetric information given in Plate 3 is a subset of this data.

Plate 9. Larger-scale bathymetry of the summit region of segment A2. Red line marks the trace of the spreading axis. double-headed arrow marks the amount of crust generated on the eastern flank within the last 2 Ma. Black box indicates the area covered by the side-scan information shown, together with an interpretation using the same symbols as Plates 3–6, at the top. See text for further discussion.

146 SOUTH ATLANTIC HYDROTHERMALISM

tion for apparently relatively recent volcanic activity on these segments (in the form of isolated volcanic cones on segment A1 (Plate 3) and recent lava flows on segment A2, an example of the latter is shown on Plate 9).

The case of segment A2 is particularly interesting. At first appearance, it seems bathymetrically to be almost as shallow (at ~2000 m axial depth) as segment A3 where Lilliput is situated. Closer inspection, however, shows that this shal-low area is situated in a rifted valley between flanks that rise somewhat higher (~1900 m), and so, although the segment

as a whole appears inflated, the local topography is that of a slow spreading ridge: higher ridge flanks and an axial valley. The rift valley is characterized by evidence of small volume eruptions (see Plate 9, top), but no large-scale “resurfacing” as seen at Turtle Pits [German et al., 2008a] or segment A3 (Plate 5). The lava flow shown in Plate 9 has been found to possess 226Ra excesses in its magmas (T. Kokfelt, per-sonal communication, 2007), suggesting that the eruption was significantly younger than 8000 years. Faulting within the lava flow indicates tectonic activity occurred after the

Plate 10. Earthquakes in the equatorial Atlantic detected by the global seismic network in the period 1965–2008 and archived in the Incorporated Research Institutions for Seismology (IRIS) system. The symbol size is proportional to earthquake magnitude. Note the paucity of epicenters on segments A2 and A3.

DEVEY ET AL. 147

volcanic eruption, but despite extensive water column and ROV-based seafloor exploration, no evidence for hydrother-mal activity, either high- or low-temperature, was found in the vicinity of this flow. We conclude from this that, although the ridge axis itself is still relatively shallow compared to the local average depth, the period of excess volcanism, which led to this anomalous axial depth is past. This implies that the heat input into the crust was higher in the past but now has waned. Looking at the off-axis bathymetry of segment A2 (Plate 9), this appears to have happened repeatedly in the past; Plate 9 shows the amount of spreading that would have taken place over the last 2 Ma at present-day rates (~17 mm a−1 half rate based on NUVEL-1A calculations [DeMets et al., 1990, 1994], which is also in agreement with the rates derived from magnetic observations [Bruguier et al., 2003]) and shows three peaks in bathymetry (reflecting three peaks in magma production rates, similar to those which have been seen elsewhere on the MAR [e.g., Cannat et al., 1999]), one approximately every 1 Ma.

With only minor magmatic activity presently occurring on A2, the inflated axis is being split by further, less magmati-cally active spreading. Much of this spreading appears to be occurring by ductile flow of the middle and lower crust; earth-quake archives (Plate 10) show no teleseismic earthquake ac-tivity on either A2 or A3 axis since records have been kept (the earthquake data plotted in Plate 10 are those available in the Incorporated Research Institutions for Seismology (IRIS) catalogue for the period 4 November 1965 to 10 May 2008). This implies that most of the crust is above the temperature (~600°c) at which brittle deformation ceases and that deeply penetrating fractures, capable of generating earthquakes that can be detected teleseismically, are not present in the crust [Phipps Morgan and Chen, 1993]. On A3, these high tem-peratures are associated with massive surface outpourings of lava, the aftermath of which is marked by diffuse hydrother-mal venting at Lilliput. On A2, we conclude that this heat is residual, left over from a period of more intense magmatic activity that ceased relatively recently (the present-day axial trough splitting the high on A2 is approximately 750 m wide; at a full spreading rate of 35 mm a−1, such a trough would take 21.4 ka to open by plate separation alone).

One implication of this is that heat does not appear to be being extracted efficiently or at high exit temperatures from these slow spreading ridge segments with a pronounced re-gional axial high (note that such segments appear only to be found along slow spreading ridges. The “inflated” portions of the East Pacific Rise [e.g., Weiland and Macdonald, 1996] are only some hundreds of meters shallower than adjacent parts of the ridge, not kilometers shallower as we find in the Atlantic). German et al. [1994] and German and Parson [1998] have previously noted a paucity of high-temperature

hydrothermal activity on the shallow Reykjanes ridge. The evidence (both seismic and field) from the Ascension region is in agreement with this, implying that shallow slow spread-ing segments have very little high-temperature hydrothermal venting at the ridge crest and can maintain high temperatures in the crust for long periods of time. Independent support for this conclusion comes from petrological modeling of the depth at which erupted magmas last fractionated [Almeev et al., 2008] based on the chemistry of the axial lavas on segments A1–A4. This modeling shows that magmas from segments A1 and A4 last fractionated well below the crust, implying deep cooling and a lack of any major shallow crustal magma chamber in which melts reside before erup-tion. Segment A3 magmas, in contrast, are always modeled to have last cooled and fractionated at very shallow depths within the crust, implying that magmas on this segment rise to high levels before starting to cool significantly. Beneath A2, although some deeper fractionation is modeled, many of the lavas also show pressures of last crystallization situated well within the crust, in accordance with our conclusion that A2, although now magmatically much less active than previ-ously, still has a warm crust.

6. A MODEL

A volcanotectonic model explaining the interplay between magmatism, tectonics, and hydrothermalism within the rift axis and reconciling the evidence from volcanology, tec-tonics, hydrothermal prospection, and teleseismic activity is shown in Plate 11. This model is similar to the model of Wilcock and Delaney [1996] in that permeability in the crust is the key feature controlling hydrothermal activity, a con-clusion also deduced from modeling magma chamber depths on the Rekjanes ridge [Chen, 2003]. In contrast to Wilcock and Delaney [1996], however, who invoked clogging of fluid pathways due to mineral precipitation as the main means of keeping permeability low, we attribute this low permeability to the high temperature of the crust and hence its generally ductile nature. The curve at the bottom of Plate 11 shows the effect of this, in that hydrothermal output (at least as high temperature or focused diffuse flow) is not proportional to crustal heat input. The model nevertheless specifically re-quires magmatic activity to drive the hydrothermal venting, in accordance with the recent conclusions of Baker [2009] and Liu and Lowell [2009]. At very high magmatic inten-sities (which lead to shallow, thickened crust), the change from brittle to generally ductile deformation of most of the crust (certainly layer 3 and possibly also layer 2B) leads to a vast reduction in crustal permeability (no brittle deep crustal faults) and limits the amount of deep circulation, which can take place. This in turn means that large amounts

148 SOUTH ATLANTIC HYDROTHERMALISM

of deep crustal heat must be extracted by conduction [see, e.g., Wilcock and Delaney, 1996]. The middle row of graphs shows what will be observed as a consequence of this at the seafloor: At low magmatic intensity (e.g., segments A1 and A4), intrusive/eruptive events are rare, and the magma vol-umes involved are relatively low. These magmatic events initiate a period of hydrothermal circulation in the crust: pathways for fluids are abundantly available as the crust is mostly spreading by brittle deformation. This circulation is, however, short-lived, and occasional ship-based observa-tions of such ridges will generally find no evidence for active hydrothermal circulation (unless by chance the observations immediately postdate an eruption/intrusion). The bounding faults of the axial valley possibly cut the entire crustal sec-tion and may provide pathways for extracting heat from the deep crust; such hydrothermal systems may be much longer-lived and not event-related, as pointed out, for example, by Wilcock and Delaney [1996]. This appears to be the situa-tion at the TAG field [see deMartin et al., 2007] and in an extreme sense also at Nibelungen [Melchert et al., 2008]. At moderate magmatic intensities (e.g., Turtle Pits area), magmatic events are frequent enough to maintain high- temperature hydrothermal circulation at all times, peaks of hydrothermal activity coincide with intrusive/eruptive events. Observers of such ridges can expect to see hydrothermalism at any visit; several distinct hydrothermal systems driven by multiple intrusions (e.g., Turtle Pits and Red Lion) may even be present. At high magmatic intensities (e.g., Lilliput), the crust is so warm that it behaves ductilely throughout almost its entire thickness. hydrothermal circulation is confined to shal-low regions, where it extracts heat escaping relatively slowly by conduction from the crust; such systems have no deep brittle fractures, and hence, there are no zones of high poros-ity that would allow water to penetrate deeply. This produces a positive feedback, as ductile crust cools more slowly and so stays warm and ductile longer. Evidence from hydrophone surveys along the northern MAR [Smith et al., 2002] sug-gests that most crustal deformation occurs in a band <15 km on either side of the axis. If the crust can maintain its mainly ductile quality throughout the time this represents (<500 ka at the spreading rates applicable to the equatorial South At-lantic), it is possible that high crustal temperatures can be maintained for a long time. Individual magmatic events at the axis of such crust are associated with major eruptions which cause short, intense bursts of surface-driven hydrothermal-ism. These produce event plumes and somewhat longer, fast spreading ridge-like, high-temperature hydrothermal systems such as those described by Wilcock and Delaney [1996], as the dike cools and its associated permeability clogs.

This model is also supported by seismic velocity studies of normal and thickened crust. We note that on segment A3

[Bruguier et al., 2003], but also at other parts of the MAR with thickened crust such as the Kolbeinsey Ridge [Kodaira et al., 1997], crustal thickening is accommodated mainly by thickening of seismic layer 3, with layer 2 generally having the same thickness (~2 km) [White et al., 1992] everywhere. As layer 3 is generally assumed to represent the gabbroic (and hence coarse-grained and slowly cooled) section of the crust, this first-order observation indicates that a larger proportion of thick crust is slowly (= conductively) cooled rather than erupted, in direct agreement with our model.

7. PREDICTIONS

With such a model, it is possible to use the axial bathym-etry (along-axis depth variations, presence, and/or width of rift valley) of as-yet unexplored regions of the crust to make some predictions about where to go for the best chance of finding active high-temperature hydrothermal systems. Gen-erally, in areas of a slow spreading ridge characterized by a deep, wide axial valley containing small linear volcanic ridges, the chances of observing magmatically driven hydro-thermal systems will be relatively low. Some areas where our model predicts a high probability of finding hydrother-mal venting are shown in Plate 12. The regions at 13°, 15°, 26°, 33°, and 37°S have bathymetric signatures similar to the Turtle Pits region. They are parts of single segments that show a marked along-axis shoaling of the axial valley to-ward the segment center. As seen at Turtle Pits (and reported for the 26°S area by Grindlay et al. [1992]), this along-axis shoaling can result in more than 1000-m depth differences between the seafloor on the within-axis high and the adjacent rift graben. According to our model, such within-axis highs should exhibit almost constant high-temperature hydrother-mal venting and represent, in our opinion, the prime South Atlantic targets for finding high-temperature hydrothermal circulation. The bathymetric anomalies at 47.5° and 51.5°S are much broader and may be associated with major crustal thickening, similar to the situation on segment A3. This type of bathymetric anomaly can be distinguished from the Turtle Pits-type within-axis high by its lack of a well-developed and deep axial valley; the neovolcanic zone on A3-like segments will be on the shallowest portion of the ridge cross section. In most cases, such broad, shallow ridges will also show no dis-cernable axial graben in adjacent parts of the same segment; the transition to a deep axial valley seems to be associated with crossing a major (first or second order) ridge disconti-nuity. On such broad, shallow segments, it is most likely that hydrothermal venting is occurring as low-temperature, dif-fuse flow. Only immediately following an eruption will such a ridge segment show high-temperature venting, which will then be intense. An initial examination of the earthquake ar-

DEVEY ET AL. 149

chives confirm the A3-like nature of these segments, at least for the most southerly target; no teleseismic earthquakes have ever been recorded between 50.5° and 52°S!

8. CONCLUSIONS

Bathymetric, side-scan, and hydrothermal surveys between 2° and 14°S on the MAR have provided ground-truthed evi-

dence and characterization of three hydrothermal systems. Of these, one (Nibelungen) is associated with a major crustal fracture; the other two (Turtle Pits and Lilliput) are related to recent volcanism. Other areas of the ridge axis that also show signs of relatively recent volcanism are, in contrast, presently not hydrothermally active. We use these regional observations to develop a model for the relationship between volcanism, tectonism, and hydrothermalism, whose defining

Plate 11. Our model of the interplay between volcanic, tectonic, and hydrothermal processes on the Mid-Atlantic Ridge (MAR). (bottom) Relationship deduced between intensity of magmatism (which is reflected in crustal thickness and hence axial depth) and hydrothermal activity. (middle) Fluid and magma discharge associated with the three scenarios “low magmatic intensity,” “moderate magmatic intensity,” and “high magmatic intensity”. (top) Resulting crustal struc-ture. Red opaque ovals are magma bodies at temperatures above the brittle-ductile transition; red translucent areas are the regions of crust warmed by the transport of magma to these bodies. Blue ovals are magma bodies that have cooled below the brittle/ductile transition.

Plate 12. Maps of several areas of the Mid-Atlantic ridge (generated using the GeoMapApp software) showing areas which we, based on the model and discussion presented here, suspect will show the repeated development of hydrother-mal systems. The large bathymetric anomalies at 47.5° and 51.5°S may only be associated with diffuse venting (see text); the other areas should show frequent high-temperature black smoker activity. The bathymetric anomalies at 26° and 33°S were investigated by Grindlay et al. [1992] and Fox et al. [1991], who showed the seafloor, in both cases, to be more than 1000 m shallower on the within-axis high (at 2400–2600 m depth) than on neighboring parts of the same segment.

DEVEY ET AL. 151

characteristic is a nonlinear relationship between heat input into the crust by magmatism and heat extracted from the crust at the axis by hydrothermalism. This nonlinear behav-ior is the result of hot thick crust not being able to fracture brittly to any great depth, which in turn hinders the forma-tion of deep, high-temperature hydrothermal circulation and so reduces the rate at which the crust (and especially the midcrust to lower crust) can be cooled. This model has im-plications for hydrothermal exploration on slow spreading ridges: deep axial valleys with sparse magmatic activity will seldom be found to have volcanically associated hydrother-mal venting (although heat may be extracted from deep hot rock or serpentinites along deep-reaching fractures), whereas within-axis segment-center highs may be continually hydro-thermally active. Very shallow segments with thick crust will show few examples of high-temperature systems and may be characterized instead by diffuse venting.

Acknowledgments. We would like to thank the captains and crews of the research vessels “Charles Darwin,” “Meteor,” and “Atalante” without whose professionalism and dedication this study could not have been carried out. The scientific crews of cruises CD169, M41/2, M62/5, M64/1, M68/1, and ATA Leg 2 shared their experience and provided data for the common inter-pretations. Maren Walter (University of Bremen) is specifically thanked for help with collating the CTD prospection data. Two anonymous reviewers provided many helpful suggestions which led to substantial improvements in the manuscript. This is Deutsche Forschungsgemeinschaft SPP1144 contribution number 41.

REFERENcES

Almeev, R., et al. (2008), depths of partial crystallization of h2O-bearing MORB: Phase equilibria simulations of basalts at the MAR near Ascension Island (7–11°S), J. Petrol., 49, 25–45.

Baker, E. T. (2009), Relationships between hydrothermal activity and axial magma chamber distribution, depth, and melt content, Geochem. Geophys. Geosyst., 10, Q06009, doi:10.1029/2009Gc002424.

Baker, E. T., et al. (1985), hydrothermal particle plumes over the southern Juan de Fuca Ridge, Nature, 316(6026), 342–344.

Bennett, S. A., et al. (2008), The distribution and stabilisation of dissolved Fe in deep-sea hydrothermal plumes, Earth Planet. Sci. Lett., 270(3–4), 157–167.

Bruguier, N. I., T. A. Minshull, and J. M. Brozena (2003), Mor-phology and tectonics of the Mid-Atlantic Ridge, 7°–12°S, J. Geophys. Res., 108(B2), 2093, doi:10.1029/2001JB001172.

cann, J. R., et al. (1997), corrugated slip surfaces formed at ridge-transform intersections on the Mid-Atlantic Ridge, Nature, 385, 329–332.

cannat, M., et al. (1999), Mid-Atlantic Ridge–Azores hotspot in-teractions: Along-axis migration of a hotspot-derived event of enhanced magmatism 10 to 4 Ma ago, Earth Planet. Sci. Lett., 173(3), 257–269.

carbotte, S. M., and d. S. Scheirer (2004), Variability of ocean crustal structure created along the global mid-ocean ridge, in Hy-drogeology of the Oceanic Lithosphere, edited by E. E. Davis and h. Elderfield, pp. 59–107, cambridge Univ. Press, cambridge.

chen, y. J. (2003), Influence of the Iceland mantle plume on crustal accretion at the inflated Reykjanes Ridge: Magma lens and low hydrothermal activity?, J. Geophys. Res., 108(B11), 2524, doi:10.1029/2001JB000816.

Connelly, D. P., C. R. German, M. Asada, K. Okino, A. Egorov, T. Naganuma, N. Pimenov, G. Cherkashev, and K. Tamaki (2007), hydrothermal activity on the ultra-slow spreading south-ern Knipovich Ridge, Geochem. Geophys. Geosyst., 8, q08013, doi:10.1029/2007Gc001652.

curewitz, d., and J. A. Karson (1997), Structural settings of hy-drothermal outflow: Fracture permeability maintained by fault propagation and interaction, J. Volcanol. Geotherm. Res., 79(3–4), 149–168.

deMartin, B. J., et al. (2007), Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge, Geology, 35(8), 711–714.

deMets, c., et al. (1990), current plate motions, Geophys. J. Int., 101, 425–478.

deMets, c., R. G. Gordon, d. F. Argus, and S. Stein (1994), Ef-fects of recent revisions to the geomagnetic reversal time scale on estimates of current plate motions, Geophys. Res. Lett., 21(20), 2191–2194.

devey, c. W., K. S. Lackschewitz and E. Baker (2005), hydro-thermal and volcanic activity found on the southern Mid-Atlantic Ridge, Eos Trans. AGU, 86(22), 209, doi:10.1029/2005EO220001.

devey, c. W., et al. (2008), Cruise Report Cruise MARSUED IV, N.O. “Atalante” Jan 2008, 96 pp. (Available at http://www.ifm-geomar.de/fileadmin/ifm-geomar/fb4/spp1144/Atalante/Leg2_Atalante.pdf )

Edmonds, h. N., et al. (2003), discovery of abundant hydrothermal venting on the ultraslow-spreading Gakkel ridge in the Arctic Ocean, Nature, 421(6920), 252–256.

Fox, et al. (1991), The Mid-Atlantic Ridge (31°S–34°30′S): Tem-poral and spatial variations of accretionary processes, Mar. Geo-phys. Res., 13, 1–20.

German, c. R., and L. M. Parson (1998), distribution of hydrothermal activity along the Mid-Atlantic Ridge: Interplay of magmatic and tectonic controls, Earth Planet. Sci. Lett., 160(3–4), 327–341.

German, c. R., et al. (1994), hydrothermal activity on the Reyk-janes Ridge: The Steinahóll vent-field at 63°06′N, Earth Planet. Sci. Lett., 121(3–4), 647–654.

German, c. R., et al. (1998), Topographic control of a dispersing hy-drothermal plume, Earth Planet. Sci. Lett., 156(3–4), 267–273.

German, c. R., et al. (2002), hydrothermal activity on the southern Mid-Atlantic Ridge, Eos Trans. AGU, 83(47), Fall Meet. Suppl., Abstract V61B-1361.

German, c. R., et al. (2008a), hydrothermal activity on the southern Mid-Atlantic Ridge: Tectonically- and volcanically-controlled venting at 4–5°S, Earth Planet. Sci. Lett., 273(3–4), 332–344.

German, c. R., et al. (2008b), hydrothermal exploration with the Au-tonomous Benthic Explorer, Deep Sea Res., Part I, 55, 203–219.

152 SOUTH ATLANTIC HYDROTHERMALISM

Grindlay, N. R., P. J. Fox, and P. R. Vogt (1992), Morphology and tectonics of the Mid-Atlantic Ridge (25°–27°30′S) from sea beam and magnetic data, J. Geophys. Res., 97, 6983–7010.

haase, K. M., et al. (2007), young volcanism and related hydro-thermal activity at 5°S on the slow-spreading southern Mid-Atlantic Ridge, Geochem. Geophys. Geosyst., 8, q11002, doi:10.1029/2006Gc001509.

haase, K. M., et al. (2009), diking, young volcanism and diffuse hydrothermal activity on the southern Mid-Atlantic Ridge: The Lilliput field at 9˚33′S, Mar. Geol., 266, 52–64, doi:10.1016/j.margeo.2009.07.012.

hey, R., et al. (2004), Tectonic/volcanic segmentation and controls on hydrothermal venting along Earth’s fastest seafloor spread-ing system, EPR 27°–32°S, Geochem. Geophys. Geosyst., 5, Q12007, doi:10.1029/2004Gc000764.

Keeton, J. A., et al. (1997), Bathymetry of the Reykjanes Ridge, Mar. Geophys. Res., 19, 55–64.

Keir, R., et al. (2008), Flux and dispersion of gases from the “drachenschlund” hydrothermal vent at 8°18′ S, 13°30′ W on the Mid-Atlantic Ridge, Earth Planet. Sci. Lett., 270(3–4), 338–348.

Kodaira, S., R. Mjelde, K. Gunnarsson, H. Shiobara, and H. Shi-mamura (1997), crustal structure of the Kolbeinsey Ridge, North Atlantic, obtained by use of ocean bottom seismographs, J. Geophys. Res., 102(B2), 3131–3151.

Koschinsky, A., et al. (2006), cruise Report M68/1 of F.S. “Meteor”.Koschinsky, A., et al. (2008), hydrothermal venting at pressure-

temperature conditions above the critical point of seawater, 5°S on the Mid-Atlantic Ridge, Geology, 36, 615–618.

Lalou, c., et al. (1996), Initial chronology of a recently discov-ered hydrothermal field at 14°45′N, Mid-Atlantic Ridge, Earth Planet. Sci. Lett., 144(3–4), 483–490.

Liu, L., and R. P. Lowell (2009), Models of hydrothermal heat out-put from a convecting, crystallizing, replenished magma cham-ber beneath an oceanic spreading center, J. Geophs. Res., 114, B02102, doi:10.1029/2008JB005846.

Macdonald, K. c., et al. (1988), A new view of the mid-ocean ridge from the behaviour of ridge-axis discontinuities, Nature, 335, 217–225.

Melchert, B., et al. (2008), First evidence for high-temperature off-axis venting of deep crustal/mantle heat: The Nibelungen hydro-thermal field, southern Mid-Atlantic Ridge, Earth Planet. Sci. Lett., 275(1–2), 61–69.

Möller, h. (2002), Magma genesis and mantle sources at the Mid-Atlantic ridge east of Ascension Island, dissertation, 133 pp., Univ. Kiel.

Ondréas, h., M. cannat, y. Fouquet, A. Normand, P. M. Sarradin, and J. Sarrazin (2009), Recent volcanic events and the distribu-tion of hydrothermal venting at the Lucky Strike hydrothermal field, Mid-Atlantic Ridge, Geochem. Geophys. Geosyst., 10, Q02006, doi:10.1029/2008Gc002171.

Phipps Morgan, J., and y. J. chen (1993), The genesis of oceanic crust: Magma injection, hydrothermal circulation, and crustal, flow, J. Geophys. Res., 98(B4), 6283–6297.

Planert, L., E. R. Flueh, and T. J. Reston (2009), Along- and across-axis variations in crustal thickness and structure at the Mid-At-

lantic Ridge at 5°S obtained from wide-angle seismic tomogra-phy: Implications for ridge segmentation, J. Geophys. Res., 114, B09102, doi:10.1029/2008JB006103.

Reston, T. J., et al. (2002), A rifted inside corner massif on the Mid- Atlantic Ridge at 5°S, Earth Planet. Sci. Lett., 200(3–4), 255–269.

Sauter, d., et al. (2002), TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: Evidence for along-axis magma distribution, Earth Planet. Sci Lett., 199(1–2), 81–85

Schulz, h. d., et al. (1999), Meteor Cruise Report, Cruise Nr. 41, 341 pp., Leitstelle METEOR, Institut für Meereskunde, Univer-sität Hamburg, Hamburg.

Smith, d. K., et al. (1995), Mid-Atlantic Ridge volcanism from deep-towed side-scan sonar images, 25-29°N, J. Volcanol. Geo-therm. Res., 67, 233–262.

Smith, d. K., M. Tolstoy, c. G. Fox, d. R. Bohnenstiehl, h. Mat-sumoto, and M. J. Fowler (2002), hydroacoustic monitoring of seismicity at the slow-spreading Mid-Atlantic Ridge, Geophys. Res. Lett., 29(11), 1518, doi:10.1029/2001GL013912.

Smith, d. K., J. Escartin, h. Schouten, and J. R. cann (2008), Fault rotation and core complex formation: Significant processes in seafloor formation at slow-spreading mid-ocean ridges (Mid-Atlantic Ridge, 13°–15°N), Geochem. Geophys. Geosyst., 9, Q03003, doi:10.1029/2007Gc001699.

Smith, W. h. F., and d. T. Sandwell (1997), Global sea floor to-pography from satellite altimetry and ship depth soundings, Sci-ence, 277(5334), 1956–1962.

Soule, S. A., d. J. Fornari, M. R. Perfit, M. A. Tivey, W. I. Ridley, and h. Schouten (2005), channelized lava flows at the East Pacific Rise crest 9°–10°N: The importance of off-axis lava transport in developing the architecture of young oceanic crust, Geochem. Geophys. Geosyst., 6, Q08005, doi:10.1029/2005Gc000912.

Weiland, c. M., and K. c. Macdonald (1996), Geophysical study of the East Pacific Rise 15°N–17°N: An unusually robust seg-ment, J. Geophys. Res., 101(B9), 20,257–20,273.

White, R. S., d. McKenzie, and R. K. O’Nions (1992), Oceanic crustal thickness from seismic measurements and rare earth ele-ment inversions, J. Geophys. Res., 97, 19,683–19,715.

Wilcock, W. S. d. (1998), cellular convection models of mid-ocean ridge hydrothermal circulation and the temperatures of black smoker fluids, J. Geophys. Res., 103(B2), 2585–2596.

Wilcock, W. S. d., and J. R. delaney (1996), Mid-ocean ridge sul-fide deposits; evidence for heat extraction from magma chambers or cracking fronts?, Earth Planet. Sci. Lett., 145(1–4), 49–64.

D. P. Connelly, National Oceanography Centre, European Way, Southampton SO14 3ZH, UK.

C. W. Devey, K. S. Lackschewitz, and B. Melchert, Leibniz Institute of Marine Sciences at the University of Kiel (IFM-GEO-MAR), Wischhofstraβe 1-3, d-24148 Kiel, Germany. (cdevey@ifm-geomar.de)

C. R. German, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.

K. M. Haase, GeoZentrum Nordbayern, Schlossgarten 5, D-91054 Erlangen, Germany.